1. Field of the Invention
The invention relates to a composition which has long-lasting and consistently good flowability and comprises a polyamide powder and a flow aid. The composition can be used for coating moldings or for the production of test specimens by laser-sintering. The composition may also be used in the production of cosmetics or coating materials.
2. Description of the Background
Polyamide powders are widely used in industry, e.g. for coating metal parts, as additives in coating materials and cosmetics, and also for the production of test specimens or small runs by rapid-prototyping using selective laser sintering. High requirements are placed upon the processability of the powders used in these applications. A particularly critical parameter of the polyamide powder is flowability. Caking within polyamide powders can lead to inhomogeneity in the resultant coating or in the test specimen. Powders must retain good flowability whatever the storage conditions, such as unopened or opened packs. Specifically in the case of opened packs, consistent processability has to be ensured irrespective of the atmospheric conditions prevailing during storage, e.g. temperature and variations in humidity.
Polyamide powders absorb water by way of hydrophilic carboxamide groups. There is a relationship between the chain lengths of the monomers used to form the polyamide and the water absorption of the resultant polyamide. For example, powders based on nylon-11 and nylon-12 have lower water absorption since they have fewer carboxamide group based on the methylene groups in the polymer, than polyamides prepared from shorter-chain monomers. Water absorption in polyamide powders leads to caking which results in poor flowability. Caking is particularly pronounced with polyamide powders produced by milling. Poor flowability in polyamide powder is accompanied by poor processability.
U.S. Pat. No. 4,334,056 describes a process for producing pulverulent coating agents based on polyamides having at least 10 aliphatically bonded carbon atoms per carboxamide group. The coating agents produced by that process can be used to produce lacquer-like coatings on metals. The methods used for this coating include fluidized-bed sintering, flame-spraying and electrostatic coating. When heated to temperatures near the film-forming temperature, the coating compositions produced by this process form coatings with a smooth surface, good edge coating, good elasticity, and excellent resistance to aqueous alkalis, with no formation of undesirable fumes. The coating powders produced in U.S. Pat. No. 4,4334,056 have a defined particle size distribution which is determined by the production conditions. The flowability of the resultant polyamides is adequate immediately after production. As is the case with prior-art polyamide powders, when the powders produced by this process are stored at varying humidity and temperature their flowability suffers due to caking.
DE 31 13 392 discloses a process for reducing the electrostatic charging of pulverulent thermoplastic polyamides. The coating powder produced from thermoplastics using this process contains small amounts of a finely divided inorganic powder which has been coated with an antistat, together with pulverulent thermoplastic polyamides. Organic ammonium compounds or organic amine compounds are present as an antistat. The amount of resultant coated powder added is from 0.01 to 0.03 part by weight, based on 100 parts by weight of polyamide. The resultant modified polyamide powder has better fluidizability in fluidized-bed sintering when compared with an unmodified powder. However, DE 31 13 392 gives no information as to whether the coating powder described retains its flowability, and thus its suitability as a coating powder, after storage at varying temperature and humidity. Since the antistat coating is hydrophilic, it may be assumed that the flowability of the coating composition is reduced on storage at varying temperature or humidity.
Petrovicova et al. [J. Appl. Polym. Sc. 77 (2000) 1684–1699; ibid. 78 (2000) 2272–2289] describe a high-velocity oxyfuel process (HVOF process). In the HVOF process a polymer powder is provided with pigments to improve the properties of the coating layer. The powder composition is used for coating metal parts. The polymer powder is melted by means of a combustion spray gun and, while molten, applied to the material to be coated. Petrovicova investigated the effect of hydrophilic silicas, hydrophobic silicas, carbon black or A 1100 γ-aminopropyltriethoxysilane-modified silica as pigments in the mixture with nylon-11 on the properties of the resultant coatings. For this, 5, 10, 15 and 20% by volume of the pigment were added to the nylon-11. This corresponds to a proportion of from 0.3 to 3% by weight of pigment in the coating composition. The respective pigment and nylon-11 are mixed and milled for 48 h in a mill which comprises zirconia beads as a milling aid. The long milling time incorporates the pigment very homogeneously into the nylon-11 grains. Petrovicova found that the properties of the resultant coating are dependent on the proportion of pigment. The highest strength properties of the resultant coating are achieved with 20% of pigment by volume in the coating composition. Neither of the documents describes improvement of the flowability of the coating composition due to the pigments used. Nor is there any indication to the skilled worker of the effect of the pigments disclosed on the flowability of the coating composition.